Hydraulic actuation systems that use hydraulic power to facilitate mechanical motion (e.g. linear, rotary or oscillatory motion) have many uses across a range of technologies. An hydraulic actuation system typically comprises an hydraulic power supply, a metering valve (e.g. an electro-hydraulic servo valve or direct drive servo valve) controlled by an electronic servo controller, and an actuator driven by the hydraulic flow from the flow metering valve. A typical linear hydraulic actuator comprises a hollow tube along which a piston can slide and can be single-acting or double-acting. In a double-acting actuator, hydraulic fluid pressure is applied from a chamber on each side of a piston, and the pressure differential between the two chambers moves the piston one way or the other.
Propeller pitch control systems commonly use hydraulic actuation systems to control the pitch of the propeller blades, known as pitch change actuators. Variable pitch propellers are employed on many different types of vehicles, such as aircraft. Typically, propeller blades are mounted to a rotary hub for pivotable movement about their longitudinal axis to permit pitch adjustment. The pitch adjustment is controlled by a linear double-acting hydraulic pitch change actuator housed within the rotating hub assembly. On one side of the piston is an “increase pitch pressure chamber” and on the other side a “decrease pitch pressure chamber”, with the differential pressure between the two moving the piston so as to cause the pitch angle to increase or decrease. The pitch change actuator is driven by a metering valve e.g. an electrohydraulic servo valve or direct drive servo valve, for pressuring the pitch change actuator chambers to effectuate a desired change in pitch of the propeller blades. The electrohydraulic servo valve or direct drive servo valve is fed by hydraulic fluid from the engine lubrication system, and is controlled by a servo controller utilising feedback from position sensors. The position sensors may be installed within the rotating propeller system, an LVDT or an RVDT can sense actuator position or blade pitch angle for instance. The positions sensors may also be installed on the engine where magnetic sensors can sense blade and propeller rotational speed targets and generate a voltage pulse stream that can be read by the engine or the aircraft control computers.
Such pitch change actuators are well known in the art, for example in U.S. Pat. No. 8,439,640B2.
In existing pitch change actuation systems, the servo controller, hydraulic power supply and metering valve are located in the static part of the nacelle, e.g. the static part of the engine driving the propeller. The servo valve delivers hydraulic fluid flow and pressure to a coarse pitch hydraulic supply line and to a fine pitch hydraulic supply line which are provided, through a hydrodynamic bearing, to the rotating part of the propeller and thereby to the actuator.
However, this transfer of hydraulic power from the static part of the nacelle to the rotating part requires the use of complex systems and a number of subsystems installed on the engine gear box such as a Propeller Control Module and high pressure pumps (main supply pump and electrical auxiliary pump for redundancy in case of failure of the main oil circuit). Reliability, maintenance, weight and performance of the pitch change actuation are impacted by the complexity of the architecture. Furthermore, the number of LRUs installed on the engine gear box increases the number of subcomponents interfacing with the engine.
In known propeller system architectures, the propeller is fed with hydraulic fluid (oil) by the engine turbomachinery lubrication circuit and the drain is returned to the engine. Therefore, propeller drain pressure is not constant and depends on engine lubrication system behaviour. The variation in drain pressure affects the accuracy of the metering valves. The air content of the oil coming from the engine is sometimes significant as this oil is also used for lubrication of turbomachinery. The air content of the oil is a well-known issue in hydraulic actuation systems which can affect the actuation performance. The oil is also subject to the pollution that could be collected by the oil along the engine lubrication system.
The hydraulic fluid (oil) used for propeller actuation is typically a lubrication oil also used for the turbomachinery lubrication, since this oil supply is already present. However, this type of oil does not have the same physical characteristics as oil which is dedicated to actuation and that is typically used in some other aircraft actuation systems (like Skydrol which has a reduced changed in viscosity versus temperature operating range).
The present disclosure seeks to address the above described issues.